Bridge Seismic Isolation Study on a Full Scale Bridge Test

Myrto Anagnostopoulou SEESL Structural and Test Engineer

Ricardo Ecker LayPh.D. Candidate

Andre Filiatrault Professor, MCEER Director

Dep. Of Civil, Structural and Environmental EngineeringUniversity at Buffalo – State University of New York

Design of seismically isolated structures is based on the mechanical properties of newly-fabricated seismic isolation hardware

Environmental effects, history of loading, aging result in change in:

properties of isolation hardware behavior of isolated structure

Collect field data on the aging characteristics and long-term service life of seismic isolation bearings

Full-Scale Isolated Bridge

Calspan’s Ashford facility, Western NY – 50 miles from UB Two 72-foot long adjacent single lane concrete girder bridges at a

distance of 6 feet 8 low-damping elastomeric bearings of two different elastomeric

compounds Free vibration testing will be repeated weekly for a period of 5 years

starting end-October 2010 Remotely controlled testing from SEESL/UB facilities

Superstructure Geometry

10 girder beams: AASHTO Box cross-section (BII-36), 70 skew 8 beams weight 26 tons and 2 beams weight 32 tons longitudinal post-tensioning at bottom plate of girder beams transverse post-tensioning of girder beams at the support sections 9” of gravel fill equivalent to 7” concrete/asphalt deck

Abutment

IsolationBearings

Girder Beams

Gravel

Spreader

Beam

ashford.wmv

Elastomeric Isolators Target period of isolated bridge: T=2 sec Total weight per bearing: W=100 kips Design deformation: D=4 in

10 low-damping elastomeric bearings of circular cross-section with two different rubber compounds:Group A -> G=120 psi -> k=2.7 kips/inGroup B -> G=70 psi -> k=1.6 kips/in

Groups A and B are assigned to each of the two adjacent bridges

Characterization testing of isolation bearings at SEESL/UB in order to acquire mechanical properties

20091015-MOV0DF.mpg

Bearing Characterization at SEESL

Group A kA=2.5 kips/in ζA=5%

Group B kB=1.5 kips/in ζB=3%

Full-Scale Bridge Testing

Actuator

LoadCell

Actuator spans the gap between the two adjacent single span bridges

Slow extension rate of the actuator up to: 16” to the reaction

load cell 4” design displacement

of the bearings Fast retraction rate of the

actuator in order to subject the two bridges in free vibration

Forcing System Properties: Max actuator stroke: 24 in Max actuator force: 50 kips

Group B

Group A

4.0 in

2.4 in

F=24kips

F=24kips

0 10 20 30 40 50 60 70 80 90 100-4

-3

-2

-1

0

1

2

3

4

Time (sec)

Dis

plac

emen

t (in

)

Group B

0 10 20 30 40 50 60 70 80 90 100-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

Time (sec)

Dis

plac

emen

t (in

)

Group A

Group A initial displacement: 2.4 in damping: 5% period: 2.0 sec free vibration duration: 35 sec number of cycles: 15

Group B

initial displacement: 4 in damping: 3% period: 2.6 sec free vibration duration: 70 sec number of cycles: 25

Testing Procedure

1. Collect the initial mechanical properties of the isolation bearings (stiffness, damping)

2. Run bridge free vibration set of tests remotely from SEESL/UB3. Collect data/info from:

actuator, reaction load cell accelerometers, load cells, string potentiometers,

thermocouples (26 sensors in total) cameras weather station

4. Obtain post-testing mechanical properties of bearings and compare to pre-testing ones

5. Visit the bridge field station in order to check condition of bearings, actuator, instrumentation

6. Repeat the procedure weekly and for a period of five years

System Property Modification Factors Properties of seismic isolation bearings:

Characteristic strength, Qd

Effective stiffness, Keff

Post-yield stiffness, Kd

Damping ratio, ζ Phenomena effecting isolator properties:

Temperature Aging Wear or Travel History of loading

Which are the max and min probable values of the bearing properties within the structure’s lifetime?

Can all phenomena occur simultaneously?

Pn

λmax= λmax,1·λmax,2·λmax,3 ···

λmin= λmin,1·λmin,2·λmin,3 ···

Bounding Analysis

λ-factors: quantify the effect of a particular phenomenon on the nominal properties of an isolation bearing

Pmin = λmin·Pn

Pmax = λmax·Pn

Pmax controls the substructure and superstructure force response Pmin controls the isolator displacement response System Property Adjustment factors account for the probability

that several events occur simultaneously, depend on the significance of the structure and their values are based on engineering judgment

According to AASHTO (1999): “The λ-factors listed herein are based on the available limited data. In some cases the factors could not be established and need to be determined by test.”

max: 800 to 1000F

min: -300 to 100F

λ-factors for Elastomeric Bearings Temperatures for design: 700F to -220F (AASHTO, 1999)

Low temperatures cause increase in stiffness and strength Duration of exposure is significant but usually neglected

Travel or Wear due to traffic and temperature changes: For a cumulative movement of 1 mile 17 sets of free

vibration tests should be conducted during one day of testing (AASHTO, 1999)

λ-factors depend significantly on the rubber compound of the bearing

λmax,t

λmax,tr

Better understanding of the effect of temperature, environmental conditions and ware on the mechanical properties of isolation bearings

Realistic determination of bounding values of isolator properties for analysis and design based on better estimated Property Modification Factors

Using different seismic isolation systems the bridge field station can provide an insight to the resilience of bridges due to naturally-occurring phenomena

Conclusions

Acknowledgments

SEESL technical staff and students

Doug Stryker and Andrew Dailey from Calspan

H&K Services for constructing the bridge

Hubbell Galvanizing for donating the girder beams

Dynamic Isolation Systems for providing the bearings

Thank you! Questions?

Instrumentation/DAQ System

actuator displacement, load cell sensors 26 sensors:

10 accelerometers 2 load cells (temperature range -10F – 100F) 10 string potentiometers 4 thermocouples

7 digital cameras 1 digital weather station 32-channel portable DAQ System compatible with existing

UB/NEES systems and software

SEESL remote desktop controller

Ashford host PC/ Pump controller

Actuator/Test

DAQ/SensorsAshford host PC

SEESL remote desktop DAQ

internet

internet

max: 80 to 100F

min: -30 to 10F

Effects on Elastomeric Bearings Wear or Cumulative Travel Temperature:

low temperatures -> increase in stiffness and strength high temperatures -> degradation of the rubber

Coupling between wear and temperature Duration of exposure and elastomeric compound control behavior Lack of long-term in-situ performance data